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Although the Government has placed work restrictions on low-skilled Bulgarians and Romanians, they can still come as visitors.

"It is quite likely that many will stay and work in the black economy," the report said.

It also anticipated that the arrival of more than 600,000 migrants from eight former Soviet bloc states since 2004 would continue to encourage others to travel to seek work.

In the short term this would lead to the workforce growing more rapidly than employment, with a consequent rise in jobless rates — though these would remain below those elsewhere in Europe.

The impact of immigration on the size of the population would also hit national wealth.

The forecasters reckoned that the population had risen by 2.5 million over the past 10 years, of which 1.7 million — or almost 70 per cent — was due to net immigration. More than one third of this increase occurred in the last two years alone.

But while the population was increasing by 0.8 per cent a year, the economy was growing by only half that rate.

''Clearly this has important implications for the potential growth rate of GDP," the report added.

However, a report yesterday from the Bank of England said it had found "little evidence" that immigrants from eastern Europe had significantly affected the wages or employment chances of British workers. The paper found that the 0.8 per cent rise in unemployment rates over the past 18 months had "little to do with" immigration.

Earlier this week, the Migrationwatch UK think-tank said that, on a per capita basis, immigration brought negligible financial benefits to the host population.

The Home Office bases its migration figures on the International Passenger Survey but this counts only people saying they intend to stay for more than a year and only arrivals at major airports. The last official figures suggested net migration had peaked and had fallen from 223,000 in 2004 to 185,000 in 2005.

But the Capital Economics report said it was unwise for business and policy-makers to rely on these statistics.

When it included workers from the eight former Soviet bloc states, the overall level of immigration rose to 780,000 in 2005 compared with a government figure of 524,000.

When those who left were subtracted from those who arrived, the report estimated net immigration for 2005 of about 400,000 — up from 295,000 a year earlier.